Process of growing a single crystal utilizing differences in chemical potential



Aug. 12, 1969 J. B. MULLIN ETA!- PROCESS OF GROWING A SINGLE CRYSTAL UTILIZING DIFFERENCES IN CHEMICAL POTENTIAL Filed March 26, 1965 2 Sheets-Sheet 1 f 7 A B FIG. I

CONCENTRATION FIG. 3.

Mao W417) M403 Attorney:

9 12. 1969 J. B. MULLIN mL 3,460,998

PROCESS OF GROWING A SINGLE CRYSTAL UTILIZING DIFFERENCES IN CHEMICAL POTENTIAL Filed March 26, 1965 2 Sheets-Sheet 7.

[Ill] Growth direction. I3

Fig. 4.7

United States Patent US. Cl. 1481.6 2 Claims The present invention relates to the production of single crystal material.

It is an object of the invention to provide a new method for the production of single crystal material. The materials concerned belong to a class including single phase materials for which it is possible either to maintain within the charge material a high strain energy or (in materials which undergo polymorphic or order/disorder transformations under certain environmental conditions) to provide a thermodynamic driving force the effect of which is to make the desired phase the stable phase under the environmental conditions. There must exist some solvent which lowers the liquidus temperature and has negligible solubility in the solid phases. An example of the first group of materials is pure or doped aluminium oxide; examples of the second group are the growth of grey tin from white tin, diamond from graphite and epsilon-cobalt from alpha-cobalt. The expression environmental conditions is here taken to include all the physical environment, for example, temperature, pressure, electric current, and magnetic and electric field.

According to the present invention there is provided a process for the growth of a homogeneous single crystal of a first material on a seed crystal of the first material from a charge of a second material which has the same chemical composition as the first material but differs structurally therefrom, the process being achieved by causing a thin liquid alloy zone sandwiched between the two materials to move through the second material, and the motion of the liquid alloy zone 'being brought about by the difference in chemical potential between the first and second material under the environmental conditions.

The second material may differ structurally from the first in that it is more highly strained or in that it is a different polymorph.

If the first and second materials are labelled A and B respectively, both having the same chemical composition X then the liquid alloy zone must be a solution of X dissolved in an appropriate solvent Y. Both X and Y can be either single chemical elements or multicomponent mixtures or compounds. Y must have the property that (i) alloys XY are molten at temperatures below the melting point of A and B at the operating pressure; (ii) Y is insoluble or virtually insoluble in solid A and B.

Growth of a single crystal of A on the single crystal A seed at the expense of the B charge is achieved by the controlled motion of the liquid alloy zone along the bar away from A in a manner described below.

Embodiments of the invention will be described by way of example and with reference to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic representation of an arrangement for the production of single crystal material;

FIGURE 2 is an isobaric surface of the phase diagram of a hypothetical binary alloy XY;

3,460,998 Patented Aug. 12, 1969 See FIGURE 3 is a graphical representation of the concentration of Y in the liquid alloy zone plotted across the length of the zone; and

FIGURE 4 is a diagrammatic representation of a process embodying the invention.

In FIGURE 1 a bar 1 consists of a seed 2 of a material A separated from a charge 3 of a material B by a sandwich 5 of alloy which is liquid at the temperature and pressure of performance of the process. The materials A and B are solid. The sandwich 5 of alloy constitutes a thin alloy zone which is caused to move in the direction shown by an arrow 7, as described below. Two embodiments of the invention will be considered.

In the first embodiment the bar is maintained at a certain uniform temperature and pressure such that A is the thermodynamically stable phase and such that some mixture of X and Y, to form the thin alloy zone, is molten. By definition, A will have a lower chemical potential than B. For a planar solid-liquid interface a gradient of chemical potential of average value (,u ;u )/L will exist across the thin liquid alloy zone, where ,u and #3 are the chemical potentials of the materials A and B respectively and L is the length of the zone measured normal to the solid/liquid interfaces. The gradient of chemical potential will cause the zone to migrate away from the seed crystal. Thus the velocity of the zone will increase with decreasing zone length and in practice the zone length is made sutficiently small for an adequate velocity to be achieved. In general L will be in the range 10 to 10- cm.

In the second embodiment A and B are different polymorphs of the same substance, and the process described in relation to the first embodiment leads to the growth of that polymorph which is the thermodynamically stable one at the operating temperature and pressure. However by providing externally another thermodynamic driving force which opposes and overrides Q/L it is possible to grow the metastable B phase at the expense of the stable A phase. Suitable external driving forces are an electric current (as described in the specification of patent application No. 19,029/63), a temperature gradient or a pressure gradient. It is assumed that the metastable phase B does not spontaneously transform to the stable phase A.

The principles will now be described with reference to FIGURE 2, which is an isobaric surface of the phase diagram of a hypotherical binary alloy XY. The substance to be crystallised exists in two polymorphic forms A and B, and the transition temperature between the forms is T The liquid Y dissolves X but is virtually insoluble in solid X. The liquidus curve 9 has a discontinuity in slope at the transition temperature T The slopes of the two portions of the liquidus curve 9 are designated m and m We consider an experiment performed in the A stable region of the phase field and to determine the condition of metastable equilibrium between B and the liquid we extrapolate the B liquidus curve in to the A phase field at 11.

Consider initially the isothermal Example 1 above, in which the sandwich is at a temperature T. If, initially, the concentration of Y in the liquid alloy zone is C then with respect to the A solid the liquid is supercooled and with respect to the B solid super-heated. The A/ liquid interface therefore freezes and the B/liquid interface dissolves until the concentration in the liquid zone reaches C at the A/liquid interface and C at the B/liquid interface. This is represented diagrammatically in FIGURE 3, where concentration is plotted against distance across the zone. Both interfaces are now in local equilibrium. This is not an equilibrium arrangement however because diffusion will take place across the liquid zone in the concentration gradient of average value (C C )/L. A steady transfer of Y across the zone from the A/liquid to the B/liquid interface occurs and zone moves along the bar away from the seed end.

If now a direct electric current is passed axially through the sandwich in that direction which produces a differential migration of Y ions towards the A/liquid interface, then for some magnitude of the current the electrostatic force on the Y ions will exactly balance the diffusion force and the concentration gradient (C C )/L will be stabilised and no zone motion will occur. A larger current will result in a steady transfer of Y across the zone from the B/liquid to the A/liquid interface up the concentration gradient and the B phase will grow at the expense of the A. Similarly if a temperature gradient is applied across the zone such that the temperatures at the A/liquid and B/liquid interfaces are respectively T and T then both interfaces are in equilibrium at the concentration C. In this case no concentration gradient exists and the zone remains stationary. The application of a steeper temperature gradient sets up a concentration gradient in the opposite sense (C C and the zone moves in the opposite direction i.e. B i grown at the expense of A.

The direction of motion of the zone is thus determined by the sign of the diiference of the opposing thermodynamic forces.

We have appreciated that in the second embodiment where the unstable phase is made to grow at the expense of the stable phase there is in the melt no gradient of constitutional supercooling with respect to the freezing interface. The absence of a gradient of constitutional supercooling is a very important factor contributing to the homogeneity and perfection of crystals grown by any process.

In order to control the pressure at which the process takes place the bar 1 may be in some pressure-controlled enclosure, for example, a pressure bomb.

The orientation of the surface seed crystal A in contact with the liquid alloy may have a significant influence on the microscopic perfection of single crystal of certain materials. For example for certain diamond cubic semiconductors it may be desirable to make the surface orientation a {111} plane under certain conditions.

By way of example, a procedure for the growth of alpha (grey) tin from beta (white) tin will be described. Consider the arrangement shown diagrammatically in FIGURE 4, in which a single crystal seed 11 of alpha tin is mounted on a refrigerated block 13. A block (or cylinder) 15 of beta tin is placed on top of the seed 11 but is separated from it by a thin film 17 of liquid mercury saturated with tin. Surface tension forces prevent the block 15 from touching the seed 11 directly. The thin liquid film 17 must wet both the surface of the seed 11 and that of the block 15 completely. An alternative arrangement can be used in which the block 15 and the seed 11 are physically separated by a thin mica annulus (as in patent specification No. 19,029/63). It is desirable to arrange that the surface of the seed 11 in contact with the liquid is a plane of the type {111} and that growth is made to proceed in the l1l direction. The growth of alpha tin occurs when beta tin dissolves in the mercury rich liquid film 17, diffuses across the 4 film and finally crystallises on the surface of the seed crystal 11.

The growth of the alpha tin is achieved by either of the two embodiments of the invention described above.

In the first case, the whole system is maintained at the temperature of the refrigerated block 13 (for example by putting it in a refrigerator). This temperature must be less than 8 C. and its optimum value lies in the region of 20 C.

In the second case, a steady temperature gradient is applied axially to the system by heating the block 15 from above with a small heater (not shown). The optimum average temperature of the liquid film may not be the temperature employed in the first case and it must be determined by experiment, but it will lie in the range 20 C. to +20 C.

The whole apparatus is surrounded by a suitable inert ambient gas, or by a vacuum.

We claim:

1. A process for the growth of a homogeneous single crystal of a first material on a seed crystal of the first material from a charge of a second material which has the same chemical composition as the first material but differs structurally therefrom in that it is a different polymorph thereof and thus has a different chemical potential, the process being achieved by causing a thin liquid alloy zone sandwiched between the two materials to move through the second material, and the motion of the liquid alloy zone being brought about by the difference in chemical potential between the first and second materials under the environmental conditions, said process being carried out substantially isothermally such that temperature gradients which might aid or deter the movement of the liquid alloy zone are substantially absent.

2. A process for the growth of a homogeneous single crystal of a first material on a seed crystal of the first material from a charge of a second material which has the same chemical composition as the first material but dilfers structurally therefrom in that it is more highly strained and thus has a different chemical potential, the process being achieved by causing a thin liquid alloy zone sandwiched between the two materials to move through the second material, and the motion of the liquid alloy zone being brought about by the difference in chemical potential between the first and second materials under the environmental conditions, said process being carried out substantially isothermally such that temperature gradients which might aid or deter the movement of the liquid alloy zone are substantially absent.

References Cited UNITED STATES PATENTS 3,378,409 4/1968 Hurle et a1. l48-l.6 2,813,048 11/1957 Pfann 1481.5 2,932,562 4/1960 Pfann 1481.6

L. DEWAYNE RUTLEDGE, Primary Examiner PAUL WEINSTEIN, Assistant Examiner US. Cl. X.R. 

1. A PROCESS FOR THE GROWTH OF A HOMOGENEOUS SINGLE CRYSTAL OF A FIRST MATERIAL ON A SEED CRYSTAL OF THE FIRST MATERIAL FROM A CHARGE OF A SECOND MATERIAL WHICH HAS THE SAME CHEMICAL COMPOSITION AS THE FIRST MATERIAL BUT DIFFERS STRUCTURALLY THEREFROM IS THAT IT IS A DIFFERENT POLYMORPH THEREOF AND THUS HAS A DIFFERENT CHEMICAL POTENTIAL, THE PROCESS BEING ACHIEVED BY CAUSING A THIN LIQUID ALLOY ZONE SANDWICHED BETWEEN THE TWO MATERIALS TO MOVE THROUGH THE SECOND MATERIAL, AND THE MOTION OF THE LIQUID ALLOY ZONE BEING BROUGHT ABOUT BY THE DIFFERENCE IN CHEMICAL POTENTIAL BETWEEN THE FIRST AND SECOND MATERIALS UNDER THE ENVIRONMENTAL CONDITIONS, SAID PROCESS BEING CARRIED OUT SUBSTANTIALLY ISOTHERMALLY SUCH THAT TEMPERATURE GRADIENTS WHICH MIGHT AID OR DETER THE MOVEMENT OF THE LIQUID ALLOY ZONE ARE SUBSTANTIALLY ABSENT. 